Wireless cellular communication networks and their operation are generally well known. In such a system the area covered by the network is divided into cells. Each cell is provided with a base station, which is arranged to communicate with a plurality of mobile stations or other user equipment in a cell associated with the base station.
In these known systems, it is possible to locate a mobile station with reference to a base station, and therefore possible to locate a mobile station within the operational transmission range of a base station.
As is also known additional location information can be determined by measuring the time between transmission and reception of a signal between a mobile station and a known base station or transmitter. Using such time of arrival (TOA) methods with signals transmitted from base stations it is possible to locate a mobile station within tens of metres.
Using the base station to transmit timing signals and using these signals to determine a positional estimate produces an estimate containing several potential errors and problems One of the major problems is the many different paths that the transmissions from the base station to the mobile station can take. The path can be direct, which provides an accurate estimation of the distance between the base and mobile stations or the path can be diffracted or reflected by man-made or natural phenomena such as buildings, large vehicles and hills. These indirect paths do not reflect the true distance between the base station and the mobile station and therefore produce location estimation errors. These diffracted and reflected signal paths occur more frequently in built-up and urban environments, thus degrading the more accurate base station location estimations due to the increased density of base stations.
A separate development in location estimation has been the development of a global positioning satellite (GPS) system which enables a GPS receiver to accurately locate its position within a few metres by measuring the transit time of signals received from satellites orbiting the earth. In basic terms the GPS system relies on both the transmitter (the orbiting satellites) and the receiver to have prior knowledge of a transmitted sequence, for their clocks to be substantially synchronized, and for the receiver to make measurements of a number of different satellite transmissions. The receiver calculates the time difference between the different satellite transmissions, and multiplying by the speed of light can calculate its own position in relation to that of the satellites. Additionally the receiver must have further information including the orbits (and hence position) of the satellites at any given time (this can be decoded from the satellite transmissions if necessary). Armed with this information, the time difference measurements and the absolute time at which the measurements are applicable, the receiver's position can be computed.
As is known in the art the GPS orbiting satellites are accurately synchronized ‘to GPS System time’ each carrying an accurate very stable atomic clock. Furthermore the constellation of satellites is monitored from controlling ground stations and any timing errors detected are effectively corrected.
As the cost of supplying each GPS receiver with an accurate and stable clock oscillator such as an atomic clock is prohibitive. Fortunately, this requirement can be mitigated, allowing the use of inexpensive oscillators. This comes at the expense of a common time error affecting the transit time measurements for all satellites equally. The common timing error is known as receiver clock bias and becomes a fourth unknown along with the 3-D position coordinates. This fourth unknown means that the receiver must therefore make measurements of a minimum of four satellite vehicles (SVs) in order to resolve both position and accurate GPS system time.
A further development in location estimation has been the development of assisted GPS (A-GPS). The idea here being that information such as the position of the satellites, rough receiver position, and fairly accurate GPS system time are passed to the receiver in order to speed up the time taken to make a position fix, and furthermore, as is known in the art, to increase the likelihood of measuring the time difference of the visible satellite transmissions and successfully obtaining a position fix at all. An example of A-GPS can be found in certain cellular networks where an entity on the cellular network side may at times provide assistance data to a mobile station.
Although the provision of accurate GPS system time in the assistance data set is highly desirable, it is likely that in many cellular networks supporting A-GPS, this particular information will not be available due to the additional network infrastructure that it implies. In this scenario it is then necessary for the mobile station's GPS receiver to decode GPS system time from a satellite data transmission. Unfortunately this activity requires higher signal strength (reducing the chances of obtaining a fix, especially in weak signal areas) and may take a number of seconds to complete. This not only contributes to the overall time to compute a positional fix, but also means that the receiver is active for longer, and hence more power is consumed. Power consumption is an important consideration in a mobile device.
As the elapsed time taken to decode and synchronize to GPS system time can be quite a few seconds (and may not even be possible in weak signal areas), it is highly desirable that once synchronization is achieved, the receiver is able to maintain GPS time even when it is not making a fix and the RF circuitry of the receiver is inactive. This is usually taken care of by a local clock oscillator and associated counter circuitry. The accuracy of the oscillator will determine for how long the local clock remains sufficiently synchronized to ‘real GPS system time’ in order to be useful.
As is mentioned above, cellular mobile stations may be equipped with GPS receiver modules in order to improve the location estimation capacity of the mobile station.
The cost sensitive nature of mobile stations tends to preclude the inclusion of expensive clock oscillators, and the type of oscillators used are by themselves too inaccurate to perform the role of maintaining synchronization to GPS System time for any length of time. With incremental errors of the order of several parts per million, sufficiently accurate synchronization will be lost in a matter of minutes.
The mobile station could of course re-acquire GPS time by turning on the GPS receiver, but to perform this exercise every few minutes would be a significant drain on the battery and somewhat negate the benefit of maintaining accurate GPS system time locally.
U.S. Pat. No. 5,945,944 describes a combined GPS receiver and mobile station transceiver, wherein the GPS timing information is determined by signals received from the base station, which is then transmitted via a communications link to be processed by a separate base unit. The positional estimate of the mobile station is not immediately available to the mobile station.
U.S. Pat. No. 6,346,911 describes a method of determining GPS time by capturing GPS data and locating a predetermined code sequence within the captured GPS data and the time difference between the captured data start and the start of the located predetermined code.
U.S. Pat. No. 6,150,980 describes a method of determining the time for a GPS receiver. Timing signals derived from a communication system are received by a GPS receiver and decoded to provide accurate time information.